1. Field of the Invention
The present invention relates to centrifugal pumps and more particularly to a shrouded inducer for use with a centrifugal pump. The present invention is more particularly directed to eliminating the cavitation damage which normally would result from a recirculation flow of fluid about the shroud of the inducer.
2. Description of the Prior Art
It has been found that the addition of the shroud to an otherwise shroudless inducer arrests the formation of vortices at or about the tips of the inducer blades and thus avoids the cavitation damage to the inducer associated with said vortices. The addition of a shroud, however, creates problems of its own in that a portion of the fluid downstream of the inducer tends to recirculate about the outer periphery of the shroud and re-enters the main flow just upstream of the inducer blades. As the recirculating fluid emerges from behind the forward or downstream edge of the shroud, it will often shed vortices which impinge directly upon the more radially outward portions of the inducer blades. These vortices create an erosive action upon the inflicted portions of the blades and ultimately result in the inducer suffering a similar loss in efficiency and structural integrity as a shroudless inducer. Thus, the use of a shroud to avoid the problems associated with blade tip vortices is compromised by the problems associated with vortices shed at the forward edge of the shroud.
Various attempts have been made to overcome the problems associated with recirculation flow about a shrouded inducer. For example, labyrinth seals have been placed about the outer periphery of the inducer shroud to minimize recirculation flow over the shroud. However, no matter how good the labyrinth seal, there is always some amount of flow which passes over the seal which will then cause the aforementioned vortices problem. Moreover, as time goes by, labyrinth seals tend to lose their sealing effectiveness, especially in pumps where vibration and thermodynamics subject the seal to any degree of rubbing. An extensive use of labyrinth seals could be employed to reduce the recirculation flow to a minimum such as is suggested in U.S. Pat. No. 2,984,189. Such an extensive use of seals, however, is impractical and costly. Various other methods have been proposed with regard to the construction of a shrouded inducer to overcome the problems associated with vortices emanating from the shroud. None of these, however, has been altogether effective or practical from a cost point of view.